1. Field of the Invention
The present invention relates to heat exchangers and, more particularly, to heat exchangers designed to preheat combustion air for a combustion furnace, using heat energy removed from the flue gas.
2. Prior Art
The thermal efficiency of combustion furnaces and combustion systems has typically been increased by recovering heat energy from the resulting flue gas and using this energy to preheat the combustion air. This preheating has been effected in a number of ways, including the use of recuperator type heat exchangers, by which thermal energy is transferred to the combustion air. These heat exchanger structures have ranged from comparatively simple devices, in which the flue gas and combustion air are carried in adjacent ducts that are in heat exchange relationship with one another, to far more sophisticated devices that include tube-and-shell heat exchangers, thermal siphons, and heat pipe type heat exchangers.
Recent increases in the cost of hydrocarbon fuels have necessitated improvement in the overall thermal efficiency of combustion furnaces. The search for these higher efficiencies has been complicated further by two factors: (1) the economic necessity of using fuels having a higher than preferred sulphur content and (2) the need for fuels requiring greater than usual quantities of combustion air to realize the full heat value of the fuel. An example of one such high sulphur fuel, requiring large amounts of combustion air, is the coal typically available in the western United States.
Prior tube-and-shell type heat exchangers, used as combustion air preheaters, have generally demonstrated adequate performance. However, these types of heat exchangers require large surface areas to effect efficient transfer. This large surface area requirement results in a cleaning and maintenance problem associated with the deposition of soot and other particles from the flue gas flows. In addition, these large surface heat exchangers are subject to corrosive attack when used in the lower temperature ranges because of acid vapor condensation. In a like manner, heat-pipe heat exchangers have also demonstrated good operating performance but their upper temperature limit of operation is generally considered low when compared to the high temperature of flue gases produced in the combustion process. The operation of heat pipes above their rated temperature limit results in performance degradation of the heat pipe and, occassionally, tube burn-out. In addition, since heat pipes operate in the lower temperature ranges of the flue gas, they are also subject to corrosive attack by acidic components of the flue gas. While high-temperature heat pipes exist and can be fabricated to withstand corrosive attack, these types of heat pipes generally require costly materials and heat transfer mediums, which are too expensive for conventional combustion air preheater applications.